Dog or Canine?

This is a pretty good one. Initially, Indigo translated it as "a dog is a dog". We discussed it a little. She thought of "canine", which is probably the closest thing you can do. What do you know, the number of letters in canine is exactly double of that in dog ;-).

儿子：犬就是狗，狗就是犬。究竟什么时候该写犬，什么时候该写狗呢？
父亲：忙的时候将狗写成犬，闲的时候那就将犬写成狗。
儿子：为什么？
父亲：狗的笔画比犬多一倍呀！

Son: "A dog is a canine, a canine is a dog. so when should I write it as dog, and when should I write it as canine?"
Father: "When you're in a hurry you should write dog, and when you're not very busy you should write it as canine."
Son: "why?"
Father: "Canine has double the letters than dog!"